A. Field of the Invention
The present invention relates to a method for reducing the blade-vortex interaction noisexe2x80x94generally referred to as BVI noise in the artxe2x80x94generated by a rotary wing. It also concerns a rotary wing implementing such a method.
B. Description of Related Art
It is known that, during low-speed flights, in particular when descending before landing, the blades of a rotor of a helicopter or of a similar aircraft move in proximity to their own wake and interact with the vortices which they have shed in the vicinity of their tip and which are generally referred to as xe2x80x9ctip vorticesxe2x80x9d. These interactions create abrupt variations in pressure on the blades; the more intense the vortex and the closer it passes to the blade, the greater the amplitude of these pressure variations. The latter are sources of loud noise in a particularly audible frequency band. Since the blade-vortex interaction noise radiates downward and forward, that is to say toward the environs of heliports, it constitutes one of the most penalizing acoustic nuisances in the development of helicopters. Its reduction is today a major industrial issue, especially on account of the strengthening of international standards relating to acoustic nuisances.
In order to reduce the intensity of BVI noise, essentially two actions are known, consisting respectively in decreasing the intensity of the blade tip vortices and in distancing said vortices from said blades. These actions may be amalgamated and implemented by passive or active means.
For example, the documents FR-A-2 636 593, EP-A-0 482 932 and WO 97/07019 describe passive means of this type, formed by geometries of blades, especially of tips of blades, intended to reduce the intensity of said vortices.
Passive means of this type are optimized for a given flight configuration. They have the advantage of a certain technological simplicity. On the other hand, their effectiveness may depend greatly on the flight configuration. Specifically, the conditions of the BVI noise change as a function of the flight speed and of the angle of descent of the helicopter. Moreover, the techniques proposed are unnecessary in flight configurations which do not generate BVI noise and then generally prove to be penalizing in respect of the aerodynamic performance of the rotor.
Active means of reducing BVI noise are for example described by the documents EP-A-0 689 990 and U.S. Pat. No. 5,588,800. The first of these documents employs air jets at the trailing edge and at the tip of the blades in order to reduce the intensity of the tip vortices and distance them from said blades. The second uses a trailing edge flap toward the tip of the blades, the deflection of said flap being periodic and defined so as to accelerate the vertical convection of the tip vortices and thus distance them from said blades.
Active means of this type are technically more complex than said passive means, but they can be adjusted as a function of the flight configuration of the aircraft. They may therefore be activated only when they are necessary and be optimized within a more extensive flight domain. On the other hand, in their manner of operation they do not comprise any continuous reactive relation between the flight configuration under actual flight conditionsxe2x80x94and their control. Furthermore, their manner of operation requires significant power. In particular, in the case of the device of document U.S. Pat. No. 5,588,800, the rotary wing has to be provided with high power, able to make the aerodynamic flaps oscillate at frequencies of the order of 20 Hz with amplitudes of several degrees.
The object of the present invention is to remedy these drawbacks. It relates to an active means of reducing BVI noise of the type with trailing edge flaps, the deflection of which can be feedback-controlled in relation to the actual flight conditions and which requires only relatively low power for its operation.
To this end, according to the invention, the method for reducing the noise generated by the rotary wing of an aircraft, such as a helicopter, due to the fact that, in the course of the rotation of said rotary wing and of the advancing of said aircraft, each component blade of said rotary wing encounters the tip vortex generated by a previous blade, said method implementing at least one trailing edge flap disposed toward the outboard tip of each of said blades, is noteworthy in that:
at least one value xcexa8o of the azimuth of the blades is determined, for which value said blades shed tip vortices responsible for a peak intensity of said noise;
the section of each blade around which the speed circulation is a maximum is determined;
said trailing edge flap is disposed on each blade in such a way that the former generates at least one auxiliary vortex, parallel to said tip vortex and attached to a section of said blade lying between said section around which the speed circulation is a maximum and the tip section of said blade; and
a deflection is applied to each of said flaps, its value being constant in azimuth, but such that, for said value xcexa8o of the azimuth, the speed circulation around said blade section to which said auxiliary vortex is attached is a specified fraction of said maximum speed circulation.
The present invention is based on the fact that BVI noise is not generated uniformly around the axis of rotation of the rotary wing, and that, during each cycle of rotation, each blade of the rotary wing sheds vortices whose characteristics, such as location of shedding, dimension, intensity, etc. are related to the load and to the geometry of the blades. The applicant has in particular found that the tip vortices of blades responsible for dominant BVI noise are shed by the bladesxe2x80x94regardless of how many blades go to make up the rotary wingxe2x80x94at a specified azimuth, generally lying between 120xc2x0 and 150xc2x0 and often roughly 130xc2x0, reckoned in the direction of rotation of the blades starting from the 0xc2x0 azimuth, which corresponds to the rear part of the longitudinal axis of the aircraft. BVI noise therefore includes, during each cycle of rotation, at least one peak intensity corresponding to the interaction with the tip vortices shed at this specified azimuth.
The value xcexa8o of the azimuth, as well as the section of each blade around which the speed circulation is a maximum can be determined by calculation or by trials.
Thus, by virtue of the present invention, at the azimuth xcexa8o and at azimuths neighboring xcexa8o, the blades of the rotary wing shed several weaker vortices than the single vortex which would be shed at blade tips in the absence of implementation of the invention. The latter therefore makes it possible to divide each of these single vortices into several vortices of lower speed circulation rather than to distance them from the following blades. This results in a reduction in a dominant part of the BVI noise.
Of course, in the case where it is necessary, it is possible to apply the method in accordance with the present invention to several values xcexa8o1, xcexa8o2, . . . xcexa8on of the azimuth at which the tip vortices shed would be the cause of BVI noise rather than to a single privileged value xcexa8o of the azimuth of the blades.
Likewise, in accordance with the present invention, it is possible to employ more than one trailing edge flap per blade, so as to divide the vortical shedding into a plurality of auxiliary vortices. However, in this case there is a risk that auxiliary vortices which are too close to one another may combine into a single vortex.
Also, in a preferred mode of implementation of the present invention:
a single trailing edge flap is employed per blade;
said section to which said auxiliary vortex is attached is located at least approximately midway between said section around which the speed circulation is a maximum and said tip section of said blade; and
the speed circulation around the blade section to which said auxiliary vortex is attached is at least approximately equal to half said maximum speed circulation.
Thus, each blade sheds, at the value xcexa8o of the azimuth and in the vicinity of this value, two vortices whose circulation is roughly half that of a single vortex at the blade tip. It has been found that this splitting of the vortical shedding into two similar vortices, in the vicinity of a single blade azimuth value xcexa8o at which the tip vortices responsible for the dominant BVI noise are shed, brings about a reduction in said BVI noise of the order of 7 dB.
Of course, said flaps may be actuated by any known mechanical, electrical, pneumatic or hydraulic means.
It will be observed that, in the method in accordance with the present invention, the deflection of the flap or flaps at the value or values xcexa8o of the azimuth of the blades depends on the load of the blades at the same azimuth values, implementing as it does the speed circulation around the sections of said blades.
Each blade of the rotary wing can comprise a trailing edge flap which extends as far as the blade tip section. If said blade includes only this one flap, the latter therefore extends between said blade tip section and the blade section located approximately midway between said section around which the speed circulation is a maximum and said blade tip section. In this case, the single trailing edge flap generates two similar vortices attached to its tips.
As a variant, each blade of the rotary wing can comprise a trailing edge flap which is separated from said blade tip section by a tip portion of said blade. If said blade includes only this one flap, said blade tip portion extends between said blade tip section and the blade section located approximately midway between said section around which the speed circulation is a maximum and said blade tip section. In this case, three vortices occur, one attached to said tip section and the other two attached to the tips of said trailing edge flap. To prevent the vortex attached to the inboard tip of the flap from interfering with the other two and/or from being the cause of noise, it is then advantageous for the span of said flap to be relatively large for example at least equal to 3 C (C being the chord of the blade airfoil), so that said inboard tip of the flap, and hence the vortex attached thereto, is spaced apart from the other two vortices, in the direction of the axis of said rotary wing.
It is however advantageous for each blade to be equipped with just one flap placed at the tip of said blade, of span lying between 0.4 C and 3 C, preferably equal to 1.2 C.
Regardless of the disposition of said flaps on the blades, it is advantageous for the depth of said flaps to be chosen between 0.1 C and 0.4 C, preferably equal to 0.25 C.
To implement the method in accordance with the present invention, it is possible to use lift information, pressure information or strain information to glean the speed circulations around the blade section at maximum circulation and around the section midway between the latter and the tip section. This information can be delivered by sensors and/or calculated by an on-board computer.
Thus, by virtue of the present invention, said flaps can easily be controlled so as to obtain a reduction in the BVI noise, at least that due to the tip vortices shed at the specified values xcexa8o. For this purpose, the angle of deflection a of said flaps can be feedback controlled.
For reasons of power, it is preferable for the feedback control to relate to the deflection a of said flaps only for a specified value xcexa8o.
Thus, a significant advantage of the present invention lies in the fact that it can implement trailing edge flaps of reduced dimensions together with low power. Specifically, the deflection of the flaps is modified only upon changes in the flight configuration. The feedback control of the flaps therefore gives rise to low-amplitude movements over relatively long times (typically several tenths of a second). This low power makes it possible to employ electric controls and articulations made of electrically deformable materials for the flaps. Finally, the flaps can be adjusted independently for each blade, therefore making it possible to circumvent any differences in their characteristics and their behavior.